Well it isn't a uniform distance from front to back. there's a little less than 1/2 inch(about 7/16) at the front tip of the fender, and at the back tip it's a little more than half an inch.
Btw, 13mm is roughly 1/2 inch, so that additional 9mm is cutting it close...too close IMO. I wouldn't want to have it any closer than what I have, but you could lengthen the fender mount bolt holes to give you more clearance. Or some other way...there's always a way to get something to fit.
